There are several online sources that give an explanation, but the information given here says that the folks at the RAE disagree with that information.
Since the RAE didn't provide an answer to your question, let's see if a native speaker has the answer.

__________________Lo propio de la verdad es que se basta a sí misma, aquel que la posee no intenta convencer a nadie."An enemy is somebody who flatters you. A friend is somebody who criticizes the living daylights out of you."